172 



STUDIES IN LUMINESCENCE. 



instrument, but being nearly parallel to the axis of the collimator. The 

 small amount of stray light from 7\ which unavoidably did reach the slit 

 was balanced by light from the second tungsten lamp T 2 as shown in the 

 figure. The results were entirely negative. Thinking that the intense 

 green line of the mercury arc might produce so much effect in weakening 

 the fluorescence in Fi that additional light from T x would produce only a 

 slight effect, we interposed in some cases blue glass as shown, and in 

 order to diminish the excitation produced by the light from T\ we some- 

 times used green glass as indicated. The results, however, were in all 

 cases negative. 



In Fig. 169 is shown an arrangement for testing the effect of transmitted 

 light upon fluorescence in case the direction of transmission is opposite to the 

 direction in which fluorescence is observed. No effect could be detected. 



In Fig. 170 are shown the essential parts of an apparatus used in applying 

 the method suggested by Wood. 1 A disk, shown in detail at the right of the 

 figure, was arranged so that the four sectors might either occupy the 

 position shown in the upper diagram, or might be shifted with reference to 

 each other so that the inner openings and the outer openings would be 



T, 

 O- 



Mercury arc 

 /2 Plane glass 



x 



Source 



=&=- 



Plane glass 



'CD t 2 

 - o 



c MgC0 3 



Screens 



Collimator 



</*-MgC0 3 

 'Ground glass 



Fig. 169. Fig. 170. 



alternate. This disk was mounted between the exciting source and the 

 fluorescent cell F, as shown. Light passed through the outer sectors to 

 excite fluorescence, and light passing through the inner sector, after reflec- 

 tion from a block of magnesium carbonate, passed through the cell to the 

 collimator slit. Having balanced the illumination against a standard 

 placed before the second collimator for the case where the inner and outer 

 sectors were open alternately, the disk was then changed so as to make the 

 excitation and transmission simultaneous. The results of several sets of 

 observations with this apparatus are given in Table 21. The observa- 

 tions indicate a slight negative effect, that is, the transmission of the solu- 

 tion appears to be greater during fluorescence. But the effect indicated is 

 so small that we are inclined to look upon it as resulting from accidental 

 errors. 



A few measurements were made by a method essentially the same as that 

 used by us in 1904, except that the source of the light transmitted was the 

 fluorescence of a portion of the solution. A dilute solution of fluorescence 

 was contained in a cell 20 cm. long, so arranged that either the half nearest 



R. W. Wood, Philosophical Magazine, p. 940, 1908. 



